I think it is easier to express the opposite of the sentence: "Buddha cannot save the unwilling."
天雨雖大,不潤無根之草; 佛門雖廣,不度無緣之人
Because of the concepts of "fate" and "destiny" do not translate well from Chinese into English, it may be better to use the expression below to relate the Buddhist idea of salvation:
Though the rain be plentiful, it waters not the rootless blades of grass.
Though the gates to Nirvana be wide, they are not open to those unwilling to enter.